Bulk Synthesis of Nanoporous Palladium and Platinum Powders

ABSTRACT

Disclosed is a method for providing nanoporous palladium and platinum powders. These materials were synthesized on milligram to gram scales by chemical reduction of tetrahalo-complexes with ascorbate in a concentrated aqueous surfactant at temperatures between −20° C. and 30° C. The prepared particles have diameters of approximately 50 nm, wherein each particle is perforated by pores having diameters of approximately 3 nm, as determined by electron tomography. These materials are of potential value for hydrogen and electrical charge storage applications.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of and discloses subjectmatter that is related to subject matter disclosed in parent applicationU.S. Ser. No. 12/371,821 (Attorney Docket No. SD-10991.1) filed Feb. 16,2009 and entitled “BULK SYNTHESIS OF NANOPOROUS PALLADIUM AND PLATINUMPOWDERS.” The present application claims the priority of its parentapplication which further claimed the priority under 35 U.S.C.§119(e)(1) of co-pending provisional application Ser. No. 61/066,398,filed Feb. 19, 2008 entitled “SYNTHESIS OF NANOPOROUS PALLADIUM.” Boththe parent application and the provisional application are incorporatedherein by reference.

STATEMENT OF GOVERNMENT INTEREST

The United States Government has a paid-up license in this invention andthe right in limited circumstances to require the patent owner tolicense others on reasonable terms as provided for by the terms ofcontract No. DE-AC04-94AL85000 awarded by the U.S. Department of Energyto Sandia Corporation.

TECHNICAL FIELD OF THE INVENTION

This present invention pertains to a method for the manufacture ofnanoporous palladium and platinum powders. More particularly, thisinvention pertains to the synthesis of palladium and platinum powdershaving particle diameters of approximately 50 nm, wherein each particleis perforated by a plurality of approximately 3 nm pores. Such materialsare of potential value for hydrogen isotope storage and electricalcharge storage applications.

BACKGROUND

Palladium and platinum are of well known value for applications incatalysis, hydrogen storage, and electrochemistry (Lewis, F. A.,International Journal of Hydrogen Energy, 1996, v.21(6): pp. 461-464).Interfacial interactions can often limit performance so a high surfacearea material is usually desired. In these cases, as a practicalmaximum, every point in the material particle would be ideally within afew atoms of an interface. Such materials would exhibit highdouble-layer capacitance, higher reaction rates in kinetically limitedinterfacial reactions, and in the case of palladium, rapid charging withhydrogen. When the hydrogen isotope is tritium, the helium decay productis more likely to diffuse out of the particle, limiting the formation ofbubbles that can compromise mechanical properties. Porous platinum andpalladium thin films have been fabricated by electrochemical depositionin a surfactant template (cf. Attard, G. S.; Bartlett, P. N.; Coleman,N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, v.278:pp. 838; Bartlett, P. N.; Gollas, B.; Guerin, S.; Marwan, J. PhysicalChemistry Chemical Physics, 2002, v.4: pp. 3835; Choi, K. S.; McFarland,E. W.; Stucky, G. D. Advanced Materials, 2003, v.15: pp. 2018; Denuault,G.; Milhano, C.; Pletcher, D. Physical Chemistry Chemical Physics, 2005,v.7: pp. 3545; Elliott, J. M.; Attard, G. S.; Bartlett, P. N.; Coleman,N. R. B.; Merckel, D. A. S.; Owen, J. R. Chemistry of Materials, 1999,v.11: pp. 3602; Elliott, J. M.; Owen, J. R., Physical Chemistry ChemicalPhysics, 2000, v.2: pp. 5653), and films of porous nickel (the remainingGroup 10 metal) have been formed by electrodeposition followed bydealloying (cf. Sun, L.; Chien, C. L.; Searson, P. C. Chemistry ofMaterials, 2004, v.16: pp. 3125). Furthermore, bulk powders of porousnickel can be formed conveniently through chemical reduction of nickelsalts around a surfactant template (cf. Yamauchi, Y.; Yokoshima, T.;Momma, T.; Osaka, T.; Kuroda, K. Journal of Materials Chemistry, 2004,v.14: pp. 2935; Yamauchi, Y.; Momma, T.; Yokoshima, T.; Kuroda, K.;Osaka, T., Journal of Materials Chemistry, 2005, v.15: pp 1987;Yamauchi, Y.; Yokoshima, T.; Momma, T.; Osaka, T.; Kuroda, K.,Electrochemical and Solid State Letters, 2005, v.8: pp. C141) and bulkplatinum and palladium nanostructures have been achieved by radiolytic(cf. Surendran, G.; Ramos, L.; Pansu, B.; Prouzet, E.; Beaunier, P.;Audonnet, F.; Remita, H., Chemistry of Materials, 2007, v.19: pp. 5045)and chemical metal reduction in soft templates (cf. Kijima, T.;Yoshimura, T.; Uota, M.; Ikeda, T.; Fujikawa, D.; Mouri, S.; Uoyama, S.,Angewandte Chemie—International Edition, 2004, v.43: pp. 228;Solla-Gullon, J.; Montiel, V.; Aldaz, A.; Clavilier, J. Journal of theElectrochemical Society, 2003, v.150, E104; Teng, X. W.; Liang, X. Y.;Maksimuk, S.; Yang, H. Small, 2006, v.2: pp. 249).

Other relevant methods include assembly of nanoparticles in a blockcopolymer (cf. Warren, S. C.; Messina, L. C.; Slaughter, L. S.;Kamperman, M.; Zhou, Q.; Gruner, S. M.; DiSalvo, F. J.; Wiesner, U.,Science, 2008, v.320: pp. 1748) and reduction in mesoporous silica (cf.Shin, H. J.; Ko, C. H.; Ryoo, R. Journal of Materials Chemistry, 2001,v.11: pp. 260; Kang, H.; Jun, Y.; Park, J. I.; Lee, K. B.; Cheon, J.,Chemistry of Materials. 2000, v.12: pp. 3530). These previous approacheshave brought disadvantages in scalability, safety, purity, versatility,and/or pore density.

We present a convenient pathway that is satisfactory in all of thesecriteria, resulting in Pd and Pt nanopowders with 2 nm-3 nm pores thatwe have produced in gram-scale batches.

SUMMARY

The invention then comprises a method for providing nanoporous palladiumand platinum powders.

It is, therefore, an object of this invention to provide palladium andplatinum powders having an average particle size of about 80 nm or less,wherein each particle comprises a plurality of nanopores approximately 2nm-3 nm in diameter.

It is also an object of this invention to provide a method for preparingnanoporous palladium and platinum powders using a reducing agent whichdoes not introduce metal boride impurities.

It is yet another object of this invention to provide a method forslowing the reaction for reducing palladium and platinum so that thereaction does not occur on the timescale of constituent mixing and sothat microscopic conditions are as uniform as possible.

Both the foregoing general description and the following detaileddescription are exemplary and explanatory only and are intended toprovide further explanation of the invention as claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention and are incorporated in and constitute part of thisspecification, illustrate several embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate one or more embodiments of the presentinvention and, together with the description, serve to explain theprinciples of the invention. The drawings are only for the purpose ofillustrating one or more preferred embodiments of the invention and arenot to be construed as limiting the invention. In the drawings:

FIG. 1A shows an Auger spectrum of a palladium sample rinsed in ethanol.

FIG. 1B shows an Auger spectrum of a palladium sample rinsed in ethanol,then in water, and then again in ethanol.

FIG. 2 illustrates a thermo-gravimetric analysis (TGA) of nanoporouspalladium in air, rinsed in room temperature ethanol and water (coldrinse, 1 K/min); and in air, rinsed in heated ethanol and water (hotrinse 2 K/min).

FIG. 3A illustrates nitrogen adsorption isotherms of palladium made bythe present method at 77 K after a degassing step at 50° C. for 5 hours.

FIG. 3B illustrates the pore size distributions of a palladium powdermade by the present method. A sharp peak below 2 nm corresponds to theporosity incorporated in the palladium particles, while the broaderdistribution of pore size reflects the spacing between particles.

FIG. 3C shows an expanded view of the pore size distribution shown inFIG. 3B in order to show details of the peak at about 2 nm whichdemonstrates that these pores are accessible to the gas.

FIG. 4 shows a transmission electron microscope (TEM) image ofnanoporous palladium powder made by the present method.

FIG. 5A-5F shows 1 nm-thick slices through the reconstruction of a3-dimensional reconstructed volume rendering of an original STEM imageof a palladium particle: all of the slices demonstrate the porosity istortuous and extends throughout the particles in all directions.

FIG. 6 shows a STEM image of nanoporous platinum powder made by theprocess of the present method.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

We describe herein a method to provide nanoporous palladium and platinumwhich avoids the shortcomings of the prior art.

Materials: Palladium black, ammonium tetrachloropalladate, ammoniumtetrachloroplatinate, and palladium(II) chloride were purchased fromAlfa (Alfa. Aesar, Ward Hill, Mass.). BRIJ® 56 (a polyethylene glycolhexadecyl ether surfactant, e.g., C₁₆H₃₃(OCH₂CH₂)_(n)OH, where n isabout 10), sodium chloride, ammonium chloride, ascorbic acid;hydrochloric acid, and ammonium hydroxide were all purchased fromAldrich (Sigma-Aldrich Co., St. Louis, Mo.). All materials were used asreceived and 18 MΩ deionized water was prepared in the laboratory.

Metal salt pastes: 17.7 mg of palladium(II) chloride (0.1 millimole) and47.0 mg of sodium chloride (0.8 millimole) were added to 0.4 mL ofdeionized water and heated to 80° C. in a water bath. All solidsdissolved, forming a brown solution. Alternatively, 28.4 mg of ammoniumtetrachloropalladate was dissolved in the same volume of water, with noadded sodium chloride, and heated to 80° C. For platinum, 37.3 mgammonium tetrachloroplatinate was dissolved in 0.3 mL water and 0.1 mL4M hydrochloric acid, and heated to 80° C. In addition, other alkalimetal ammonium chlorides or bromides and their complexes with palladiumor platinum can be used. In each case, 0.66 mL of BRIJ® 56, previouslymelted in the water bath, was added to the solution and mixed by shakingor preferably with a vortex mixer until the paste becomes too viscous toagitate easily. Heating and mixing cycles (typically 3) were continueduntil a homogenous brown paste is formed. The paste was left on thebench to cool to room temperature, and then placed in a −20° C. freezer.

Reducing paste: 14.4 μL concentrated ammonium hydroxide (about 30%,0.214 millimole) and 41.9 mg ascorbic acid (0.238 millimole) or asimilar formulation of buffered ascorbate were dissolved in 0.186 mLwater, and heated to 80° C. 0.33 mL BRIJ® 56 was added, and the mixturehomogenized and cooled as above, resulting in a white paste.

Porous metal particles: Pastes cooled to −20° C. were kneaded togetherwith ceramic spatulas for several minutes in a casserole dish that alsohad been cooled to −20° C. No color change in the pastes was seen atthis point. Equal amounts of the kneaded paste were loaded into two 50mL centrifuge tubes and returned to the freezer. One day later, thetubes were moved to a 4° C. refrigerator, and left for 2 days. Thepastes were stirred and then left for 1 day at room temperature, andgradually turned black over these 4 days. In the case of platinum, nocooling below room temperature was necessary, and the preferred reactiontime was 2 weeks.

To remove the surfactant and byproducts, the tubes were filled withethanol and heated to 80° C. with intermittent vortexing and sonication.This dissolved the paste, leaving a black suspension. Solids wereseparated by centrifugation at 6000 rpm, and the faintly yellowsupernatant was decanted. The black material was resuspended in 50 mLwater, heated to 80° C. for 10 minutes, and centrifuged again. Thematerial easily resuspends in water, so it was decanted carefully,leaving approximately 1 mL. This step was repeated one more time withethanol. Higher yields can be obtained using preferably two rinses of asolution comprising 3 parts (by volume) ethanol to 1 part (by volume)water. The procedure has been tested on scales ranging from 0.1millimole to 5.5 millimoles. We have been able to purify up to 1millimole per centrifuge tube.

In this approach, metal salts in the aqueous phase are reduced to metalparticles that grow around hexagonally packed surfactant cylinders. Whenthe surfactant is washed away, a pore remains. Careful choice ofreducing agent and conditions allows extended growth of particleswithout disturbing the surfactant structure and minimizes incorporationof impurities.

At room temperature, mixing the palladium and ascorbate pastes resultsin immediate reaction; the mixture turns gray or black in patches. It ispreferable to slow the reaction so that it does not occur on thetimescale of mixing and that microscopic conditions are as uniform aspossible. Adding 8 equivalents of sodium or ammonium chloride to thepalladium paste (or 6 to the palladium paste and 3 to the ascorbatepaste) slows the reaction rate sufficiently. This can also be achievedby reducing the temperature. The results reported here use bothapproaches although it is preferable to use primarily the latter,because it results in less material that must be removed during theisolation steps.

The reaction is exothermic, so care must be taken during mixing to keepthe temperature low. For scales much larger than 1 gram, the use ofchloride to slow the reaction may be preferable, but we obtainedsatisfactory results by kneading on a cold, nonporous surface. Platinummust be kept acidic to suppress reaction with oligo(ethylene oxide) atthe elevated temperatures needed to prepare the paste. Under theseconditions, reduction by ascorbic acid is slow, so added salt andreduced temperature are less important. The palladium reaction also canbe performed under acidic conditions without cooling below roomtemperature, but it proceeds more quickly at higher pH. Keeping the pHnear the pK_(a) of ascorbate helps ensure that the reaction rates at thestart and end of the reaction are comparable, allowing high yield to beachieved in a relatively short time.

Cleaning the product is challenging given the high surface area, poreaspect ratio, and reactivity of a bare palladium surface. Ethanolefficiently removes nearly all of the surfactant, but washing with water(or an ethanol-water mixture) is required to remove salts, asillustrated in the Auger spectra shown in FIGS. 1A and 1B. When washesare performed at room temperature, thermogravimetric analysis shows masslosses in the 1% range under air at 180° C.-200° C., suggesting thepresence of organic contaminants. This number is several times smallerif the washing steps are heated as shown in FIG. 2. As a point ofreference, if pores are assumed to be spaced by one diameter, one wouldexpect about 20% of the material's volume to be pores, and palladium is10 times denser than typical organic material, so clogged pores would beexpected to show a 2% mass loss in the air analysis.

BET surface area measurements, shown in FIG. 3A, were performed with anASAP 2020 Accelerated Surface Area and Porosimetry Analyzer (obtainedfrom Micromeritics Instrument Corporation, Norcross, Ga.) using nitrogenas the analytic gas at 77 K. Prior to analysis, samples were de-gassedunder vacuum for several hours at 50° C. FIG. 3B shows the pore sizedistributions of a quantity of palladium powder made by the presentmethod, wherein the sharp peak below 2 nm corresponds to the porosityincorporated in the palladium particles. The broader distribution ofpore size reflects the spacing between particles. FIG. 3C shows anexpanded view of the same pore distribution up to 20 nm and more clearlyshows the 2 nm peak. The presence of this peak demonstrates that thepores are accessible to the measuring gas.

Pore geometry can be elucidated using transmission electron microscopyand tomography. FIG. 4 shows particles imaged at zero tilt angle,showing faceted particles in the 50 nm range that are perforated by 2-3nm pores arranged with a degree of regularity and density but notclose-packed.

To demonstrate that the high-contrast regions shown in FIG. 4 are poresthat go through the entire particle, scanning transmission electronmicrographs were collected at several angles and reconstructed to form athree-dimensional representation of several particles. A series of 1 nmslices through this reconstructed view are shown in FIGS. 5A-5F, andshow pores that nm the length of the particle, with slightirregularities in their path.

FIG. 6 shows a micrograph of platinum particles produced by theprocedure described in the experimental section, showing a greaterdegree of pore regularity. Palladium produced under the more acidicconditions used for platinum looks essentially the same as in FIG. 4.

Several effects may contribute to the observed pore density differencesbetween the two metals. Electroplated palladium in purified surfactantshows regular pores, but the more constrained environment of a film maymake regularity easier to maintain than in a free particle. We suspectthat the surfactant assembly is perturbed by concentration gradients ofreactants and products, which are exacerbated by higher reaction rates;recall that the platinum reaction proceeds much more slowly. Specificinteractions between the particle surface and the products orsurfactants may influence the arrangement of surfactant molecules, aneffect that would be more pronounced in a suspended particle than afilm. The difference between our palladium and other results also maydepend on the aspect ratio of the product. The tilted platinum particlesin FIG. 6 suggest a thinner product than the palladium particles, as isconsistent with other reports. We can expect pore regularity to be morelikely over shorter distances. If true, this effect may mean thatattempts to grow particles larger than about 50 nm will produce lessregular pores, resulting in a practical upper limit on particle size. Byseeding particle growth by mixing a half batch of previously formedparticles into the metal paste before adding the reducing paste, we wereable to produce 80 nm diameter particles with similar pore size anddensity to the seed particles, so we have not yet observed such an upperlimit.

The material presented herein is practical to prepare at useful scalesand is of potential benefit to hydrogen isotope storage, electricalenergy storage, and catalysis applications due to the high exposedsurface area provided by the observed high density of small pores. Weanticipate that surfactant templates such as BRIJ® 56 may result in thehighest practical surface areas for porous noble metals: for smallerpores, high pore surface energy and mobility may result in pore collapseat low temperatures. Results reported here show that the 2 nm-3 nm poresin palladium and platinum are stable at 80° C.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the disclosures hereinare exemplary only and that various other alternatives, adaptations, andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited to the specificembodiments as illustrated herein, but is only limited by the followingclaims.

Finally, to the extent necessary to understand or complete thedisclosure of the present invention, all publications, patents, andpatent applications mentioned herein are expressly incorporated byreference therein to the same extent as though each were individually soincorporated.

1. A method for providing a finely divided platinum metal powder,comprising the steps of: a. forming a first paste, comprising a firstaqueous mixture of an ammonium or an alkali metal tetrachloroplatinate,hydrochloric acid and a polyethylene glycol hexadecyl ether surfactant;b. forming a second paste comprising a second aqueous mixture ofammonium hydroxide and ascorbic acid or similar buffered ascorbateformulation and a polyethylene glycol hexadecyl ether surfactant; c.kneading the first and second pastes together on a surface to form amixed paste; d. forming a liquid suspension of the reacted mixed pastein a heated polar solvent; e. separating the liquid suspension into aprecipitated solid and a supernatant; f. recovering the precipitatedsolid; g. drying the precipitated solid in a vacuum or an inertatmosphere thereby providing a finely divided powder of platinum metal.2. The method of claim 1, wherein the step of forming the first pastefurther comprises the steps of: a. dissolving a first quantity of anammonium or an alkali metal tetrachloroplatinate into a first volume ofwater together with more than one equivalent of hydrochloric acid toprovide a first aqueous mixture; b. heating the first aqueous mixture toabout 80° C. to form a first solution; c. melting a first quantity of apolyethylene glycol hexadecyl ether surfactant and adding it to thefirst solution to provide a second aqueous mixture; d. mixing the secondaqueous mixture to form a first paste; e. heating the first paste toabout 80° C. to form a viscous liquid; f. repeating steps d.) and e.)until the viscous liquid is rendered homogeneous.
 3. The method of claim1, wherein the step of forming the second paste further comprises thesteps of: a. dissolving a first quantity of ammonium hydroxide andascorbic acid or similar buffered ascorbate formulation into a secondquantity of water to provide a third aqueous mixture; b. heating thethird aqueous mixture to about 80° C.; c. melting a quantity of apolyethylene glycol hexadecyl ether surfactant and adding it to thethird aqueous mixture to form a reductant mixture; d. mixing thereductant mixture to form a second paste; e. heating the second paste toabout 80° C. to form a viscous reducing liquid; f. repeating steps d.)and e.) until the viscous reducing liquid is rendered homogeneous. 4.The method of claim 1, wherein the step of forming the mixed pastefurther comprises the step of kneading the first and second pastetogether on a surface to form a mixed paste.
 5. The method of claim 1,wherein the step of recovering a metal powder comprising the steps of:a. adding a quantity of ethanol to the mixed paste and heatingethanol/mixed paste combination to about 80° C.; b. mixing and agitatingthe ethanol/mixed paste combination to dissolve the mixed pasteproviding thereby a liquid suspension; c. separating the liquidsuspension into a precipitated solid and a supernatant; d. decanting thesupernatant; e. resuspending the precipitated solid in water and heatingthe aqueous suspension to about 80° C.; f. repeating steps c.) and d.);g. repeating steps a.) through d.); h. drying the precipitated solidunder a vacuum or an inert atmosphere thereby providing a finely dividedpowder of platinum metal.
 6. The method of claim 1, wherein the step ofrecovering a metal powder comprises the steps of: a. adding a quantityof a solvent solution comprising ethanol and water to the mixed pasteand heating this solvent solution-mixed paste combination to about 80°C.; b. mixing and agitating the solvent solution-mixed paste combinationto dissolve the mixed paste thereby providing a liquid suspension; c.separating the liquid suspension into a precipitated solid and asupernatant; d. decanting the supernatant; e. repeating steps a.)through d.); f. drying the precipitated solid optionally under a vacuumor an inert atmosphere thereby providing a finely divided powder ofplatinum metal.
 7. The method of claim 6, wherein the solvent solutioncomprises 3 parts by volume ethanol to 1 part by volume water.